Halftone

The use of a printing screen is a method of pre-press in the printing technology. The glass engraving raster, which was invented by George Meis Bach in Munich in 1881, allowed for the first time to raster halftone images with the aid of photography.

In printing machines for technical reasons, usually only a specific, limited selection of colors are used, which are then printed exclusively in pure form. Most printing processes can therefore, without semitones, except for the depth-variable or two-dimensional depth-variable gravure printing or the NIP method. The problem is that at a location, either a point of color can be printed or not. It can not be controlled mostly whether little or much ink is printed at an image point. As a solution, texts, pictures and graphics - unless they are made ​​exclusively from the pure inks - rasterized to represent different shades of gray or shades. In order to represent mixtures between the inks or different brightness, this very fine pressure points of the colors are printed side by side in order to achieve the desired resultant color impression.

Dithering or rasterizing image data are converted into specified print data. Here, from halftone binary information are calculated: "print ( 1) or not (0) Print ". The impression of color and gray scale values ​​is achieved by a suitable arrangement of screen dots.

Were up in the 1980s raster effects produced by glass engraving screen or special screen film films used today special imagesetter or laser imager that work with a specially developed software. These are (short RIP) called Raster Image Processor. When the screening process generated data - for example, in PostScript or PDF files and font files - converted into printable pixel data. Depending on the technical requirements, it is possible or necessary to use various types of screening.

• 3.1 grid calculation

General Information on printing screen

Covering an area of ​​, for example, 16 × 16 imagesetter a halftone dot is assembled. The more imagesetter be on this 16er square, the darker affects the body. However, there are only less Belichterpunkte included in the square, the place from a distance, the viewer looks bright

Grid point grid cell and grid size

The grid size is the distance between the individual grid cells. Hence the grid frequency is calculated as the inverse of the screen width. The number of grid cells per unit distance is measured in " lines per inch " or " lines per inch " (lines per inch, lpi).

Typical values ​​in offset printing are 54-120 lines / cm (about 135 to 266 lpi) and newspaper printing 48 to 60 lines / cm. On an imagesetter with 2540 dpi or 1000 ppcm: 1000/16 = 62.5 ⇒ 60 screen ( ppcm = pixels per inch ). During the exposure the individual grid points from multiple laser spots are composed.

Since the size of the reproducible in print halftone dots are technical limits, especially the lowest tones in the popular screen sizes are not reproducible. Another problem is the unwanted coupling ( confluence ) of adjacent grid points ( point circuit), which can lead to a sudden increase in the Tonwertdichte where actually a steady increase is desired.

Screen angle

Periodic grid are arranged in a certain angle. With the combination of several color separations, it may happen that the various grid generate moiré effects due to unfavorable screen angle.

If the four- color printing, in which several screen angles must be printed over each other to represent a color image, one tries to control this effect by various angulations of the color separations. Common screen angle of the four-color offset printing are according to DIN 16 547 for a

• Yellow = 0 °, cyan = 75 °, black = 135 °, magenta = 15 ° or
• Yellow = 0 °, cyan = 15 °, black = 45 °, magenta = 75 °

After teaching and working book "Fundamentals of print and digital media " is the screen angle:

• Yellow = 0 °, cyan = 75 °, magenta = 45 °, black = 15 °

Fine grid

All scanning from 100 AM screening and fine, as well as the FM screens are referred to as fine- grid. This generic term facilitates the printing customers to order a photo-realistic printing, without having to deal with the advantages and disadvantages of each method. The usual requirements are:

• The halftone dots are not visible to the viewer with the naked eye
• PSO - conformity,
• No subject moiré,
• No rosette formation,
• Smooth technical grid
• Repeatability,
• No additional costs.

Modulation

Basically, amplitude modulated ( autotypical height) and frequency modulated (stochastic height) can be distinguished method. In amplitude modulation screening for generating different tone values ​​, the size of frequency-modulated screening in the number of halftone dots per screen cell is varied.

Amplitude modulated screening

Periodic method autotypically: The area is divided into a fixed number of grid cells (eg, 60 screen: 60 × 60 cells per cm ²). The variation of brightness and color impression is mainly on the size of a dot in the cell ( the amplitude). With the size of the dots to their form, for example from a positive point of the circle on the barrel shape to a square changes ( " cross-ply ", Shade 50 % ), above, the form returns as negative points to, until a complete coverage ( masstone ) is reached. By elliptical- shaped chain instead of circular - square grid structures of the sudden end point is at 50 % ( eg 40 % and 60 %) on two spaced tones, so that the tone jumps and stalls where the impact point circuit. Another way of damping is the extraction of individual pixels from the grid point to place it elsewhere ( dithering). For this purpose, the area surrounding the grid point must be included in the calculation ( supercell ).

The figure at right shows two examples with other dot shapes. Halftone image above is decomposed into squares whose size is based on the gray value. The screen angle, that is the direction of the smallest spacing of the squares in relation to the reading direction, of the upper part image is 0 °. A scanning angle of 45 ° provides generally pleasing results, see Part image below.

This can be of four or more colors form printed images, the individual color separations ( color separations ) are arranged in different screen angles to each other ( see above). This form the grid points in the interaction is known as rosette pattern that can interfere with coarse grids and superimpose also periodically structured motifs (eg in textile fabrics ) results in a gain. This is reflected in a disturbing interference patterns ( moiré ). Therefore AM screens are in problematic motifs replaced by frequency-modulated screens.

Frequency modulated screening

Cross Modulated grid

Hybrid screens: These modern screening methods unite in methods of AM and FM screening. They have their origin in the flexographic printing, detached at its photopolymer letterpress printing plates AM halftone dots in bright tones ( lights ) can not be made arbitrarily small, without breaking away. Therefore there a particular grid point size is not exceeded and for the number of grid points is reduced in the lights ( thinned ). This method provides a continuous transition from the AM screening (mainly in the midtones ) for FM screening ( in highlights and shadows ) is and has now been adapted for use in offset printing.

Another method is the same in all tonal dots size and shape ( which is based on the form of a 50 % AM tone dot itself ) in stochastically determined positions and with increasing frequency to place. The brightness levels are then obtained from thinning to complete overlap. In the offset printing practice solutions can be found that move between these two extreme methods, so that the dot shape is deliberately changed.

The error diffusion, a mixture of the latter method and dithering is used in ink-jet printers for use. Them it is not possible to form printed dots of different pitch diameters; they are technology-related only able, though tiny, but only to put equal pressure points. Therefore they rule not different sized pressure points in an ordered grid, but distribute equal pressure points randomly on the medium to be printed. Note: Some ink jets are capable of differently sized ink drop, and thus to print printing dots with different pitch diameters. For example, Epson printer thanks to piezo technology spend 2-8 picoliters large ink droplets through the nozzle. The piezoelectric crystal controls the amount of the ink liquid, which flows into the pressure chamber. The greater the amount of ink, the greater the ink drops.

Intaglio printing screen

Because of the topography of the printing plate, the low pressure method requires some compromise in screening technology. Basically, a gravure printing plate (consisting of a copper cylinder with a chrome-plated surface ) and raster elements or so-called wells on cells. They are filled with thinly liquid paint and empty in direct contact with the paper. To form a liquid-tight cups, a completely closed wall ( bars ) should be established. For this reason, the font in the low pressure is rasterized what a " malpractice " would in any other printing process. The modulation, ie controlling the amount of ink, via the variation of the Näpfchenvolumens. For this purpose there are three classical methods:

In practice, the three last-mentioned methods are combined.

Generation of the grid

Grid can be generated using photographic methods ( analog) or electronic ( digital). Analogous methods are Distanzrasterung and rasterization using a contact screen in the process camera or in the contact device. Both methods, however, are now only very rarely encountered. The electronic scanning is generated by a so-called raster image processor (RIP). The data transfer to the RIP is usually done as a PostScript file.

The most widely used today is the printing raster PostScript raster. It is used by almost all printing presses as well as laser and inkjet printers. In this color gradations of individual colors are generated by the ink is divided into a grid of lines to each other at right angles and lines of individual points. The brighter the single color to be printed, the smaller the diameter of the respective printing point: the desired brightness of the color is determined by the diameter of the individual, generally circular or elliptical pressure points. This is modeled on the photographic grid. In the conventional four-color printing, the height of the different colors are printed over each other in an angular offset of 30 °.

Grid calculation

The page to be rastered is decomposed into points that correspond to the smallest possible spot size of the output device. For a 600 - dpi laser printer, for example, such a point is 1/600 inch ( inches) tall. For screening of halftones rectangular regions of points are grouped into grid cells whose size is determined by the chosen grid size. Each cell corresponds to a grid point. The shape of the grid points is determined at the screening ( halftoning ) by point functions (spot Functions) or threshold fields (Threshold arrays). Point functions are small PostScript language procedures ( Note: PostScript is a programming language ) that bring the dot positions within a grid cell in a brightness ranking. Threshold fields contain a separate threshold for each position in the grid cell. If the brightness value of the brighter side, the pressure point is white, he is dark, he is black. Both grid techniques decide for each pressure point using the color value of the page at this point, if a trigger point is black or white.

This calculation leads to an important consequence for the prepress: Even at the edge of a grid cell of a bright image region can be black individual pressure points when the page right here has a very dark color. For images that contain fine details with sharp contrasts, such as scanned line drawings, it may be independent of the color model of the image make sense of this in a very high resolution, up to the full print resolution (eg 1270 dpi ) or greater. In comparison, normal photographic motifs, it is sufficient if the image resolution of half the grid width corresponds, ie approximately 300 ppi at a screen width of 60 lpcm ( = 153 lpi).